The present invention relates to road motor vehicles provided with a system for steering whereof steering of all the steerable wheels is controlled by an actuator specific thereto. For example, of the known electrical steering systems, there exist some which have no mechanical coupling between the steerable wheels and the steering wheel and similarly no mechanical coupling for synchronizing the steering between the steerable wheels themselves.
Electrical steering systems are known which have an individual electrical actuator for each steerable wheel, the steering system being capable of steering each of the steerable wheels selectively at an angle specific thereto, with the compatibility of the different steering actions being ensured by the steering electronics. The electrical actuator for each wheel has the function of imposing the steering angle selected by the steering electronics on the wheel in question. The steering control means available to the driver of the vehicle may be a traditional steering wheel or a lever of the joystick type or any other suitable device. The commands given by the driver of the vehicle through his or her control device are sent to the steering electronics, which is loaded with suitable programs so that the actuator or actuators can be steered appropriately.
One of the advantages of this technology is that it is ideally suited to cooperating with electronics and IT systems whereof the progress allows more and more sophisticated automatic control steps to be performed, which makes it possible for steering of the wheels to be put under the control not only of manual operation but also of a safety system. Thus, for example, it is possible to put the steerable wheels at an angle which takes account not only of operation by the driver of the vehicle but also of the observed dynamic parameters of the vehicle.
Thanks to the electrical steering, a far greater range of possibilities opens up for acting on the stability of the trajectory of a vehicle. For example, whereas at present an automatic system for correcting the trajectory of the vehicle imposes corrective yaw moments by means of the brakes of one or more wheels, going over to electrical controls of the different functions of a vehicle would make it possible to act on steering of the different steerable wheels of the vehicle to correct its trajectory.
However, the system for steering a vehicle is a vital, critical function for safety, like the brakes. Consequently, so that it can replace the mechanical steering, power-assisted or otherwise, which is used almost universally nowadays on all road vehicles, it is essential for an electrical steering system to be extremely safe. This is why electrical systems are generally designed with redundancy with the aim of ensuring that the system always remains operational even if one of its components fails. This is the concept of fault tolerant system. Thus, for example, the electrical parts of the wheel actuators are preferably redundant. In this regard, the reader is referred for example to U.S. patent application Ser. No. 2003/0098197, which gives an example of a redundant system for an electrical steering control.
However, the possibility that an essential component will fail can never be completely ruled out. For example, an electrical wheel actuator may become stuck in a certain angular position, or indeed it may undergo electrical failure so that it adopts a neutral angular position, in which case it is incapable of transmitting steering forces (or forces to maintain a straight line).
The object of the invention is therefore, in the event of complete failure of an electrical actuator acting on one of the steerable wheels, to enable control of the trajectory of the vehicle to be maintained as far as possible as its driver wishes, at least so that the vehicle can be parked in a safe place for its passengers.